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Re: [Xen-devel] How to optimize pre-copy algorithm of xen to minimize downtime?



On 11/12/12 14:33, digvijay chauhan wrote:

Hello,
If I want to optimize the performance of precopy algorithm so that live migration of virtual machine using xen occurs with minimum downtime,then how to do it?
Having spent quite a bit of time studying the time that a domain is down during migration in XenServer, I'm not convinced pre-copy is what is the biggest part of downtime, assuming the Open Source product behaviour is at least mostly doing the same things - although what the host and guest is up to before it gets suspended/paused will have a big impact as well. If the guest is VERY busy, you may well end up with very little memory that doesn't need to be copied in the final copy - not sure how you can improve this.

Have you made some measurements to show where the time is spent during downtime. What workloads are you looking at? What downtime numbers have you got at present, and what portion of this is down to actually copy time, and what is other things that need to happen during migration (e.g. bringing the virtual HD and Net devices down, and then up again on the new guest)?

My above comments assume there is a FAST link between the old and new host (my testing has been primarily using "localhost" migration, so the new guest is on the same machine as the old one). For a slow link, you may find that the copy time is more of the whole time, but again, I'm not sure there is a huge amount that can be done without a rather large amount of added complexity - and that may well be better put elsewhere, e.g. compression on the network link, or some such.

I have patch for Linux 3.7rcX that improves the time Dom0 spends mapping in the guest memory, which does have a good impact on the overall copy time, but not that huge impact on the downtime (since the copy time is only small part of overall downtime, at least in my testcases). Look at list archives for Friday 7th of Dec for the latest version.

--
Mats

regards,
Digvijay



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